In broadcasting environments and other radio systems, analog signals are transmitted across a radio interface, and the transmission of data occurs on individually assigned frequency channels. Due to the use of neighboring channels, strong interference can arise. This is especially true of land-based systems, in which the existing or future network planning tolerates neighboring channels on which the transmission is 35 dB stronger than in the desired channel. Neighboring analog channels usually result in frequency-selective interference in the desired channel. In most digital radio technologies, functions can be implemented in a channel simulator to detect such frequency-selective interference to improve the error correction possibilities. In situations where the neighboring channel is a digital channel, relying on the continual introduction of new digital multiplex technologies, it is increasingly more likely in the future that the interference will be in the nature of a Gaussian interference, i.e., an interference which can no longer be reduced by the aforesaid channel simulator technologies.
FIG. 3 illustrates a typical implementation of a receiver for broadcast signals. A high-frequency (HF) signal s is received on a line 10, and input to a low noise amplifier (LNA) 10. The amplified signal is input to a tracking filter 14, whose output signal is provided to a mixer 16. An oscillator 18 provides an oscillator output signal to the tracking filter 14 and the mixer 16. The resultant mixed signal output by the mixer 16 is input to a sawtooth filter 20, SAW, whose output signal is provided to an amplifier 22. The amplifier 22 provides an intermediate frequency signal IF on a line 24. This first subassembly from the interface to the amplifier 22 constitutes a known tuner 26. The intermediate frequency signal IF on the line 24 is input to a subsequent assembly for further processing, which is usually a demodulator 28. The intermediate frequency signal IF is presented to an analog/digital converter (ADC), 30, which converts the analog intermediate frequency signal IF on the line 24 into a digital signal on a line 32. The digital data on the line 32 is input to a digital signal processing (DSP) unit 34 which usually includes, in particular, a digital demodulation system.
Known solutions for improving the capability with regard to neighboring channels that are implemented in such arrangements can be divided into three categories.
According to the first principle, the linearity of the input stage of the tuner 26 is improved. When strong signals are present, the combined power of all these channels is taken to the amplifier stage, i.e., the low noise amplifier 12, of the tuner. This low noise amplifier 12 is usually optimized for high linearity. A receiver is known from U.S. 2002008788 in which a bias current of the high-frequency amplifier is used to increase the linearity when signals from neighboring channels are present. Thus, there are possibilities for reducing the current when no neighboring interference signals are present. High linearity of the low noise amplifier 12 prevents saturation of this stage. However, the interfering signal is not removed and the remaining portion of the system must still deal with the high interference signals.
The second principle consists of performing an additional filtering in the analog region in order to further suppress the signal components of the unwanted channel prior to the sampling in the digital region. This principle is usually employed in situations in which reception with high capability is desired, wherein the further filtering is done behind the downstream converting stage, i.e., behind the mixer 6. However, the addition of a corresponding SAW filter 20 in the analog region is very expensive. Because of the necessary steep damping curve, sawtooth filters (SAW) are used. Because of the physical dimensions, one cannot expect the price of these filters to drop significantly in the future. Furthermore, these filters produce a heavy attenuation of the desired signal, which requires a compensation by an additional amplification with the amplifier 22.
The third principle consists of tuning the tuner frequency in order to shift the unwanted interference signals as much as possible outside of the visible region of the demodulator circuit 28, which further processes the incoming signal. This accomplishes a rather large attenuation of the signal of the unwanted neighboring channel by the available filters in the system, as is known from WO 0106768. However, shifting of the tuner frequency is helpful only in situations where a single neighboring interference signal exists. Moreover, the possible shifting range is limited, or else the signal of the desired channel would also be damped by the filters in the system.
EP 1384314 teaches a frequency conversion by undersampling, involving a two-phase sampling system for reduced frequency and power requirements in regard to the amplifier and the analog/digital converter. TW 484315 teaches a television receiver for digital signals with offset tuning capabilities, in which the tuner frequency is shifted when a neighboring channel is present, and corresponding information is obtained from a channel list. KR 2000045148 teaches a device for reducing interference from a neighboring channel in a digital television receiver.
There is a need for a system and method of processing the data of a signal in order to reduce unwanted data from a neighboring channel.